Rear electrode structures for electrophoretic displays

ABSTRACT

Novel addressing schemes for controlling electronically addressable displays include a scheme for rear-addressing displays, which allows for in-plane switching of the display material. Other schemes include a rear-addressing scheme which uses a retroreflecting surface to enable greater viewing angle and contrast. Another scheme includes an electrode structure that facilitates manufacture and control of a color display. Another electrode structure facilitates addressing a display using an electrostatic stylus. Methods of using the disclosed electrode structures are also disclosed. Another scheme includes devices combining display materials with silicon transistor addressing structures.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of U.S. Ser. No. 09/141,448,filed Aug. 27, 1998, which is a continuation-in-part of U.S. Ser. No.08/504,896, filed Jul. 20, 1995, U.S. Ser. No. 08/983,404, filed Jul.19, 1997, and U.S. Ser. No. 08/935,800, filed Sep. 23, 1997, and whichclaims priority to U.S. S. No. 60/057,133 filed Aug. 28, 1997, U.S. S.No. 60/057,716, filed Aug. 28, 1997, U.S. S. No. 60/057,122, filed Aug.28, 1997, U.S. S. No. 60/057,798, filed Aug. 28, 1997, U.S. S. No.60/057,799 filed Aug. 28, 1997, U.S. S. No. 60/057,163 filed Aug. 28,1997, U.S. S. No. 60/057,118, filed Aug. 28, 1997, U.S. S. No.60/059,358, filed Sep. 19, 1997, U.S. S. No. 60/059,543 filed Sep. 19,1997, U.S. S. No. 60/065,529, filed Nov. 18, 1997, U.S. S. No.60/065,630 filed Nov. 18, 1997, U.S. S. No. 60/065,605 filed Nov. 18,1997, U.S. S. No. 60/066,147, filed Nov. 19, 1997, U.S. S. No.60/066,245, filed Nov. 20, 1997, U.S. S. No. 60/066,246, filed Nov. 20,1997, U.S. S. No. 60/066,115 filed Nov. 21, 1997, U.S. S. No. 60/066,334filed Nov. 21, 1997, U.S. S. No. 60/066,418 filed Nov. 24, 1997, U.S. S.No. 60/070,940 filed Jan. 9, 1998, U.S. S. No. 60/071,371 filed Jan. 15,1998, U.S. S. No. 60/072,390 filed Jan. 9, 1998, U.S. S. No. 60/070,939filed Jan. 9, 1998, U.S. S. No. 60/070,935 filed Jan. 9, 1998, U.S. S.No. 60/074,454, filed Feb. 12, 1998, U.S. S. No. 60/076,955 filed Mar.5, 1998, U.S. S. No. 60/076,959 filed Mar. 5, 1998, U.S. S. No.60/076,957 filed Mar. 5, 1998, U.S. S. No. 60/076,978 filed Mar. 5,1998, U.S. S. No. 60/078,363 filed Mar. 18, 1998, U.S. S. No. 60/083,252filed Apr. 27, 1998, U.S. S. No. 60/085,096 filed May 12, 1998, U.S. S.No. 60/090,223 filed Jun. 22, 1998, U.S. S. No. 60/090,232 filed Jun.22, 1998, U.S. S. No. 60/092,046 filed Jul. 8, 1998, U.S. S. No.60/092,050 filed Jul. 8, 1998, and U.S. S. No. 60/093,689 filed Jul. 22,1998, the contents of all of which are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to addressing apparatus forelectronic displays and, in particular, to rear electrode structures forencapsulated electrophoretic displays.

BACKGROUND OF THE INVENTION

[0003] Traditionally, electronic displays such as liquid crystaldisplays have been made by sandwiching an optoelectrically activematerial between two pieces of glass. In many cases each piece of glasshas an etched, clear electrode structure formed using indium tin oxide.A first electrode structure controls all the segments of the displaythat may be addressed, that is, changed from one visual state toanother. A second electrode, sometimes called a counter electrode,addresses all display segments as one large electrode, and is generallydesigned not to overlap any of the rear electrode wire connections thatare not desired in the final image. Alternatively, the second electrodeis also patterned to control specific segments of the displays. In thesedisplays, unaddressed areas of the display have a defined appearance.

[0004] Electrophoretic display media, generally characterized by themovement of particles through an applied electric field, are highlyreflective, can be made bistable, and consume very little power.Encapsulated electrophoretic displays also enable the display to beprinted. These properties allow encapsulated electrophoretic displaymedia to be used in many applications for which traditional electronicdisplays are not suitable, such as flexible displays. Theelectro-optical properties of encapsulated displays allow, and in somecases require, novel schemes or configurations to be used to address thedisplays.

SUMMARY OF THE INVENTION

[0005] An object of the invention is to provide a highly-flexible,reflective display which can be manufactured easily, consumes little (orno in the case of bistable displays) power, and can, therefore, beincorporated into a variety of applications. The invention features aprintable display comprising an encapsulated electrophoretic displaymedium. The resulting display is flexible. Since the display media canbe printed, the display itself can be made inexpensively.

[0006] An encapsulated electrophoretic display can be constructed sothat the optical state of the display is stable for some length of time.When the display has two states which are stable in this manner, thedisplay is said to be bistable. If more than two states of the displayare stable, then the display can be said to be multistable. For thepurpose of this invention, the term bistable will be used to indicate adisplay in which any optical state remains fixed once the addressingvoltage is removed. The definition of a bistable state depends on theapplication for the display. A slowly-decaying optical state can beeffectively bistable if the optical state is substantially unchangedover the required viewing time. For example, in a display which isupdated every few minutes, a display image which is stable for hours ordays is effectively bistable for that application. In this invention,the term bistable also indicates a display with an optical statesufficiently long-lived as to be effectively bistable for theapplication in mind. Alternatively, it is possible to constructencapsulated electrophoretic displays in which the image decays quicklyonce the addressing voltage to the display is removed (i.e., the displayis not bistable or multistable). As will be described, in someapplications it is advantageous to use an encapsulated electrophoreticdisplay which is not bistable. Whether or not an encapsulatedelectrophoretic display is bistable, and its degree of bistability, canbe controlled through appropriate chemical modification of theelectrophoretic particles, the suspending fluid, the capsule, and bindermaterials.

[0007] An encapsulated electrophoretic display may take many forms. Thedisplay may comprise capsules dispersed in a binder. The capsules may beof any size or shape. The capsules may, for example, be spherical andmay have diameters in the millimeter range or the micron range, but ispreferably from ten to a few hundred microns. The capsules may be formedby an encapsulation technique, as described below. Particles may beencapsulated in the capsules. The particles may be two or more differenttypes of particles. The particles may be colored, luminescent,light-absorbing or transparent, for example. The particles may includeneat pigments, dyed (laked) pigments or pigment/polymer composites, forexample. The display may further comprise a suspending fluid in whichthe particles are dispersed.

[0008] The successful construction of an encapsulated electrophoreticdisplay requires the proper interaction of several different types ofmaterials and processes, such as a polymeric binder and, optionally, acapsule membrane. These materials must be chemically compatible with theelectrophoretic particles and fluid, as well as with each other. Thecapsule materials may engage in useful surface interactions with theelectrophoretic particles, or may act as a chemical or physical boundarybetween the fluid and the binder.

[0009] In some cases, the encapsulation step of the process is notnecessary, and the electrophoretic fluid may be directly dispersed oremulsified into the binder (or a precursor to the binder materials) andan effective “polymer-dispersed electrophoretic display” constructed. Insuch displays, voids created in the binder may be referred to ascapsules or microcapsules even though no capsule membrane is present.The binder dispersed electrophoretic display may be of the emulsion orphase separation type.

[0010] Throughout the specification, reference will be made to printingor printed. As used throughout the specification, printing is intendedto include all forms of printing and coating, including: premeteredcoatings such as patch die coating, slot or extrusion coating, slide orcascade coating, and curtain coating; roll coating such as knife overroll coating, forward and reverse roll coating; gravure coating; dipcoating; spray coating; meniscus coating; spin coating; brush coating;air knife coating; silk screen printing processes; electrostaticprinting processes; thermal printing processes; and other similartechniques. A “printed element” refers to an element formed using anyone of the above techniques.

[0011] This invention provides novel methods and apparatus forcontrolling and addressing particle-based displays. Additionally, theinvention discloses applications of these methods and materials onflexible substrates, which are useful in large-area, low cost, orhigh-durability applications.

[0012] In one aspect, the present invention relates to an encapsulatedelectrophoretic display. The display includes a substrate and at leastone capsule containing a highly-resistive fluid and a plurality ofparticles. The display also includes at least two electrodes disposedadjacent the capsule, a potential difference between the electrodescausing some of the particles to migrate toward at least one of the twoelectrodes. This causes the capsule to change optical properties.

[0013] In another aspect, the present invention relates to a coloredelectrophoretic display. The electrophoretic display includes asubstrate and at least one capsule containing a highly-resistive fluidand a plurality of particles. The display also includes coloredelectrodes. Potential differences are applied to the electrodes in orderto control the particles and present a colored display to a viewer.

[0014] In yet another aspect, the present invention relates to anelectrostatically addressable display comprising a substrate, anencapsulated electrophoretic display adjacent the substrate, and anoptional dielectric sheet adjacent the electrophoretic display.Application of an electrostatic charge to the dielectric sheet ordisplay material modulates the appearance of the electrophoreticdisplay.

[0015] In still another aspect, the present invention relates to anelectrostatically addressable encapsulated display comprising a film anda pair of electrodes. The film includes at least one capsule containingan electrophoretic suspension. The pair of electrodes is attached toeither side of the film. Application of an electrostatic charge to thefilm modulates the optical properties.

[0016] In still another aspect, the present invention relates to anelectrophoretic display that comprises a conductive substrate, and atleast one capsule printed on such substrate. Application of anelectrostatic charge to the capsule modulates the optical properties ofthe display.

[0017] In still another aspect the present invention relates to a methodfor matrix addressing an encapsulated display. The method includes thestep of providing three or more electrodes for each display cell andapplying a sequence of potentials to the electrodes to control movementof particles within each cell.

[0018] In yet another aspect, the present invention relates to a matrixaddressed electrophoretic display. The display includes a capsulecontaining charged particles and three or more electrodes disposedadjacent the capsule. A sequence of voltage potentials is applied to thethree or more electrodes causing the charged particles to migrate withinthe capsule responsive to the sequence of voltage potentials.

[0019] In still another aspect, the present invention relates to a rearelectrode structure for electrically addressable displays. The structureincludes a substrate, a first electrode disposed on a first side of thesubstrate, and a conductor disposed on a second side of the substrate.The substrate defines at least one conductive via in electricalcommunication with both the first electrode and the conductor.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention is pointed out with particularity in the appendedclaims. The advantages of the invention described above, together withfurther advantages, may be better understood by referring to thefollowing description taken in conjunction with the accompanyingdrawings. In the drawings, like reference characters generally refer tothe same parts throughout the different views. Also, the drawings arenot necessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention.

[0021]FIG. 1A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich the smaller electrode has been placed at a voltage relative to thelarge electrode causing the particles to migrate to the smallerelectrode.

[0022]FIG. 1B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich the larger electrode has been placed at a voltage relative to thesmaller electrode causing the particles to migrate to the largerelectrode.

[0023]FIG. 1C is a diagrammatic top-down view of one embodiment of arear-addressing electrode structure.

[0024]FIG. 2A is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerassociated with the larger electrode in which the smaller electrode hasbeen placed at a voltage relative to the large electrode causing theparticles to migrate to the smaller electrode.

[0025]FIG. 2B is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerassociated with the larger electrode in which the larger electrode hasbeen placed at a voltage relative to the smaller electrode causing theparticles to migrate to the larger electrode.

[0026]FIG. 2C is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerdisposed below the larger electrode in which the smaller electrode hasbeen placed at a voltage relative to the large electrode causing theparticles to migrate to the smaller electrode.

[0027]FIG. 2D is a diagrammatic side view of an embodiment of arear-addressing electrode structure having a retroreflective layerdisposed below the larger electrode in which the larger electrode hasbeen placed at a voltage relative to the smaller electrode causing theparticles to migrate to the larger electrode.

[0028]FIG. 3A is a diagrammatic side view of an embodiment of anaddressing structure in which a direct-current electric field has beenapplied to the capsule causing the particles to migrate to the smallerelectrode.

[0029]FIG. 3B is a diagrammatic side view of an embodiment of anaddressing structure in which an alternating-current electric field hasbeen applied to the capsule causing the particles to disperse into thecapsule.

[0030]FIG. 3C is a diagrammatic side view of an embodiment of anaddressing structure having transparent electrodes, in which adirect-current electric field has been applied to the capsule causingthe particles to migrate to the smaller electrode.

[0031]FIG. 3D is a diagrammatic side view of an embodiment of anaddressing structure having transparent electrodes, in which analternating-current electric field has been applied to the capsulecausing the particles to disperse into the capsule.

[0032]FIG. 4A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich multiple smaller electrodes have been placed at a voltage relativeto multiple larger electrodes, causing the particles to migrate to thesmaller electrodes.

[0033]FIG. 4B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display inwhich multiple larger electrodes have been placed at a voltage relativeto multiple smaller electrodes, causing the particles to migrate to thelarger electrodes.

[0034]FIG. 5A is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display havingcolored electrodes and a white electrode, in which the coloredelectrodes have been placed at a voltage relative to the white electrodecausing the particles to migrate to the colored electrodes.

[0035]FIG. 5B is a diagrammatic side view of an embodiment of arear-addressing electrode structure for a particle-based display havingcolored electrodes and a white electrode, in which the white electrodehas been placed at a voltage relative to the colored electrodes causingthe particles to migrate to the white electrode.

[0036]FIG. 6 is a diagrammatic side view of an embodiment of a colordisplay element having red, green, and blue particles of differentelectrophoretic mobilities.

[0037] FIGS. 7A-7B depict the steps taken to address the display of FIG.6 to display red.

[0038] FIGS. 8A-8D depict the steps taken to address the display of FIG.6 to display blue.

[0039] FIGS. 9A-9C depict the steps taken to address the display of FIG.6 to display green.

[0040]FIG. 10 is a perspective embodiment of a rear electrode structurefor addressing a seven segment display.

[0041]FIG. 11 is a perspective embodiment of a rear electrode structurefor addressing a three by three matrix display element.

[0042]FIG. 12 is a cross-sectional view of a printed circuit board usedas a rear electrode addressing structure.

[0043]FIG. 13 is a cross-sectional view of a dielectric sheet used as arear electrode addressing structure.

[0044]FIG. 14 is a cross-sectional view of a rear electrode addressingstructure that is formed by printing.

[0045]FIG. 15 is a perspective view of an embodiment of a control gridaddressing structure.

[0046]FIG. 16 is an embodiment of an electrophoretic display that can beaddressed using a stylus.

DETAILED DESCRIPTION OF THE INVENTION

[0047] An electronic ink is an optoelectronically active material whichcomprises at least two phases: an electrophoretic contrast media phaseand a coating/binding phase. The electrophoretic phase comprises, insome embodiments, a single species of electrophoretic particlesdispersed in a clear or dyed medium, or more than one species ofelectrophoretic particles having distinct physical and electricalcharacteristics dispersed in a clear or dyed medium. In some embodimentsthe electrophoretic phase is encapsulated, that is, there is a capsulewall phase between the two phases. The coating/binding phase includes,in one embodiment, a polymer matrix that surrounds the electrophoreticphase. In this embodiment, the polymer in the polymeric binder iscapable of being dried, crosslinked, or otherwise cured as intraditional inks, and therefore a printing process can be used todeposit the electronic ink onto a substrate. An electronic ink iscapable of being printed by several different processes, depending onthe mechanical properties of the specific ink employed. For example, thefragility or viscosity of a particular ink may result in a differentprocess selection. A very viscous ink would not be well-suited todeposition by an inkjet printing process, while a fragile ink might notbe used in a knife over roll coating process.

[0048] The optical quality of an electronic ink is quite distinct fromother electronic display materials. The most notable difference is thatthe electronic ink provides a high degree of both reflectance andcontrast because it is pigment based (as are ordinary printing inks).The light scattered from the electronic ink comes from a very thin layerof pigment close to the top of the viewing surface. In this respect itresembles an ordinary, printed image. Also, electronic ink is easilyviewed from a wide range of viewing angles in the same manner as aprinted page, and such ink approximates a Lambertian contrast curve moreclosely than any other electronic display material. Since electronic inkcan be printed, it can be included on the same surface with any otherprinted material, including traditional inks. Electronic ink can be madeoptically stable in all display configurations, that is, the ink can beset to a persistent optical state. Fabrication of a display by printingan electronic ink is particularly useful in low power applicationsbecause of this stability.

[0049] Electronic ink displays are novel in that they can be addressedby DC voltages and draw very little current. As such, the conductiveleads and electrodes used to deliver the voltage to electronic inkdisplays can be of relatively high resistivity. The ability to useresistive conductors substantially widens the number and type ofmaterials that can be used as conductors in electronic ink displays. Inparticular, the use of costly vacuum-sputtered indium tin oxide (ITO)conductors, a standard material in liquid crystal devices, is notrequired. Aside from cost savings, the replacement of ITO with othermaterials can provide benefits in appearance, processing capabilities(printed conductors), flexibility, and durability. Additionally, theprinted electrodes are in contact only with a solid binder, not with afluid layer (like liquid crystals). This means that some conductivematerials, which would otherwise dissolve or be degraded by contact withliquid crystals, can be used in an electronic ink application. Theseinclude opaque metallic inks for the rear electrode (e.g., silver andgraphite inks), as well as conductive transparent inks for eithersubstrate. These conductive coatings include semiconducting colloids,examples of which are indium tin oxide and antimony-doped tin oxide.Organic conductors (polymeric conductors and molecular organicconductors) also may be used. Polymers include, but are not limited to,polyaniline and derivatives, polythiophene and derivatives,poly3,4-ethylenedioxythiophene (PEDOT) and derivatives, polypyrrole andderivatives, and polyphenylenevinylene (PPV) and derivatives. Organicmolecular conductors include, but are not limited to, derivatives ofnaphthalene, phthalocyanine, and pentacene. Polymer layers can be madethinner and more transparent than with traditional displays becauseconductivity requirements are not as stringent.

[0050] As an example, there are a class of materials calledelectroconductive powders which are also useful as coatable transparentconductors in electronic ink displays. One example is Zelec ECPelectroconductive powders from DuPont Chemical Co. of Wilmington, Del.

[0051] Referring now to FIGS. 1A and 1B, an addressing scheme forcontrolling particle-based displays is shown in which electrodes aredisposed on only one side of a display, allowing the display to berear-addressed. Utilizing only one side of the display for electrodessimplifies fabrication of displays. For example, if the electrodes aredisposed on only the rear side of a display, both of the electrodes canbe fabricated using opaque materials, because the electrodes do not needto be transparent.

[0052]FIG. 1A depicts a single capsule 20 of an encapsulated displaymedia. In brief overview, the embodiment depicted in FIG. 1A includes acapsule 20 containing at least one particle 50 dispersed in a suspendingfluid 25. The capsule 20 is addressed by a first electrode 30 and asecond electrode 40. The first electrode 30 is smaller than the secondelectrode 40. The first electrode 30 and the second electrode 40 may beset to voltage potentials which affect the position of the particles 50in the capsule 20.

[0053] The particles 50 represent 0.1% to 20% of the volume enclosed bythe capsule 20. In some embodiments the particles 50 represent 2.5% to17.5% of the volume enclosed by capsule 20. In preferred embodiments,the particles 50 represent 5% to 15% of the volume enclosed by thecapsule 20. In more preferred embodiments the particles 50 represent 9%to 11% of the volume defined by the capsule 20. In general, the volumepercentage of the capsule 20 that the particles 50 represent should beselected so that the particles 50 expose most of the second, largerelectrode 40 when positioned over the first, smaller electrode 30. Asdescribed in detail below, the particles 50 may be colored any one of anumber of colors. The particles 50 may be either positively charged ornegatively charged.

[0054] The particles 50 are dispersed in a dispersing fluid 25. Thedispersing fluid 25 should have a low dielectric constant. The fluid 25may be clear, or substantially clear, so that the fluid 25 does notinhibit viewing the particles 50 and the electrodes 30, 40 from position10. In other embodiments, the fluid 25 is dyed. In some embodiments thedispersing fluid 25 has a specific gravity matched to the density of theparticles 50. These embodiments can provide a bistable display media,because the particles 50 do not tend to move in certain compositionsabsent an electric field applied via the electrodes 30, 40.

[0055] The electrodes 30, 40 should be sized and positionedappropriately so that together they address the entire capsule 20. Theremay be exactly one pair of electrodes 30, 40 per capsule 20, multiplepairs of electrodes 30, 40 per capsule 20, or a single pair ofelectrodes 30, 40 may span multiple capsules 20. In the embodiment shownin FIGS. 1A and 1B, the capsule 20 has a flattened, rectangular shape.In these embodiments, the electrodes 30, 40 should address most, or all,of the flattened surface area adjacent the electrodes 30, 40. Thesmaller electrode 30 is at most one-half the size of the largerelectrode 40. In preferred embodiments the smaller electrode isone-quarter the size of the larger electrode 40; in more preferredembodiments the smaller electrode 30 is one-eighth the size of thelarger electrode 40. In even more preferred embodiments, the smallerelectrode 30 is one-sixteenth the size of the larger electrode 40. Itshould be noted that reference to “smaller” in connection with theelectrode 30 means that the electrode 30 addresses a smaller amount ofthe surface area of the capsule 20, not necessarily that the electrode30 is physically smaller than the larger electrode 40. For example,multiple capsules 20 may be positioned such that less of each capsule 20is addressed by the “smaller” electrode 30, even though both electrodes30, 40 are equal in size. It should also be noted that, as shown in FIG.1C, electrode 30 may address only a small corner of a rectangularcapsule 20 (shown in phantom view in FIG. 1C), requiring the largerelectrode 40 to surround the smaller electrode 30 on two sides in orderto properly address the capsule 20. Selection of the percentage volumeof the particles 50 and the electrodes 30, 40 in this manner allow theencapsulated display media to be addressed as described below.

[0056] Electrodes may be fabricated from any material capable ofconducting electricity so that electrode 30, 40 may apply an electricfield to the capsule 20. As noted above, the rear-addressed embodimentsdepicted in FIGS. 1A and 1B allow the electrodes 30, 40 to be fabricatedfrom opaque materials such as solder paste, copper, copper-cladpolyimide, graphite inks, silver inks and other metal-containingconductive inks. Alternatively, electrodes may be fabricated usingtransparent materials such as indium tin oxide and conductive polymerssuch as polyaniline or polythiopenes. Electrodes 30, 40 may be providedwith contrasting optical properties. In some embodiments, one of theelectrodes has an optical property complementary to optical propertiesof the particles 50.

[0057] In one embodiment, the capsule 20 contains positively chargedblack particles 50, and a substantially clear suspending fluid 25. Thefirst, smaller electrode 30 is colored black, and is smaller than thesecond electrode 40, which is colored white or is highly reflective.When the smaller, black electrode 30 is placed at a negative voltagepotential relative to larger, white electrode 40, the positively-chargedparticles 50 migrate to the smaller, black electrode 30. The effect to aviewer of the capsule 20 located at position 10 is a mixture of thelarger, white electrode 40 and the smaller, black electrode 30, creatingan effect which is largely white. Referring to FIG. 1B, when thesmaller, black electrode 30 is placed at a positive voltage potentialrelative to the larger, white electrode 40, particles 50 migrate to thelarger, white electrode 40 and the viewer is presented a mixture of theblack particles 50 covering the larger, white electrode 40 and thesmaller, black electrode 30, creating an effect which is largely black.In this manner the capsule 20 may be addressed to display either a whitevisual state or a black visual state.

[0058] Other two-color schemes are easily provided by varying the colorof the smaller electrode 30 and the particles 50 or by varying the colorof the larger electrode 40. For example, varying the color of the largerelectrode 40 allows fabrication of a rear-addressed, two-color displayhaving black as one of the colors. Alternatively, varying the color ofthe smaller electrode 30 and the particles 50 allow a rear-addressedtwo-color system to be fabricated having white as one of the colors.Further, it is contemplated that the particles 50 and the smallerelectrode 30 can be different colors. In these embodiments, a two-colordisplay may be fabricated having a second color that is different fromthe color of the smaller electrode 30 and the particles 50. For example,a rear-addressed, orange-white display may be fabricated by providingblue particles 50, a red, smaller electrode 30, and a white (or highlyreflective) larger electrode 40. In general, the optical properties ofthe electrodes 30, 40 and the particles 50 can be independently selectedto provide desired display characteristics. In some embodiments theoptical properties of the dispersing fluid 25 may also be varied, e.g.the fluid 25 may be dyed.

[0059] In other embodiments the larger electrode 40 may be reflectiveinstead of white. In these embodiments, when the particles 50 are movedto the smaller electrode 30, light reflects off the reflective surface60 associated with the larger electrode 40 and the capsule 20 appearslight in color, e.g. white (see FIG. 2A). When the particles 50 aremoved to the larger electrode 40, the reflecting surface 60 is obscuredand the capsule 20 appears dark (see FIG. 2B) because light is absorbedby the particles 50 before reaching the reflecting surface 60. Thereflecting surface 60 for the larger electrode 40 may possessretroflective properties, specular reflection properties, diffusereflective properties or gain reflection properties. In certainembodiments, the reflective surface 60 reflects light with a Lambertiandistribution The surface 60 may be provided as a plurality of glassspheres disposed on the electrode 40, a diffractive reflecting layersuch as a holographically formed reflector, a surface patterned tototally internally reflect incident light, a brightness-enhancing film,a diffuse reflecting layer, an embossed plastic or metal film, or anyother known reflecting surface. The reflecting surface 60 may beprovided as a separate layer laminated onto the larger electrode 40 orthe reflecting surface 60 may be provided as a unitary part of thelarger electrode 40. In the embodiments depicted by FIGS. 2C and 2D, thereflecting surface may be disposed below the electrodes 30, 40 vis-à-visthe viewpoint 10. In these embodiments, electrode 30 should betransparent so that light may be reflected by surface 60. In otherembodiments, proper switching of the particles may be accomplished witha combination of alternating-current (AC) and direct-current (DC)electric fields and described below in connection with FIGS. 3A-3D.

[0060] In still other embodiments, the rear-addressed display previouslydiscussed can be configured to transition between largely transmissiveand largely opaque modes of operation (referred to hereafter as “shuttermode”). Referring back to FIGS. 1A and 1B, in these embodiments thecapsule 20 contains at least one positively-charged particle 50dispersed in a substantially clear dispersing fluid 25. The largerelectrode 40 is transparent and the smaller electrode 30 is opaque. Whenthe smaller, opaque electrode 30 is placed at a negative voltagepotential relative to the larger, transmissive electrode 40, theparticles 50 migrate to the smaller, opaque electrode 30. The effect toa viewer of the capsule 20 located at position 10 is a mixture of thelarger, transparent electrode 40 and the smaller, opaque electrode 30,creating an effect which is largely transparent. Referring to FIG. 1B,when the smaller, opaque electrode 30 is placed at a positive voltagepotential relative to the larger, transparent electrode 40, particles 50migrate to the second electrode 40 and the viewer is presented a mixtureof the opaque particles 50 covering the larger, transparent electrode 40and the smaller, opaque electrode 30, creating an effect which islargely opaque. In this manner, a display formed using the capsulesdepicted in FIGS. 1A and 1B may be switched between transmissive andopaque modes. Such a display can be used to construct a window that canbe rendered opaque. Although FIGS. 1A-2D depict a pair of electrodesassociated with each capsule 20, it should be understood that each pairof electrodes may be associated with more than one capsule 20.

[0061] A similar technique may be used in connection with the embodimentof FIGS. 3A, 3B, 3C, and 3D. Referring to FIG. 3A, a capsule 20 containsat least one dark or black particle 50 dispersed in a substantiallyclear dispersing fluid 25. A smaller, opaque electrode 30 and a larger,transparent electrode 40 apply both direct-current (DC) electric fieldsand alternating-current (AC) fields to the capsule 20. A DC field can beapplied to the capsule 20 to cause the particles 50 to migrate towardsthe smaller electrode 30. For example, if the particles 50 arepositively charged, the smaller electrode is placed a voltage that ismore negative than the larger electrode 40. Although FIGS. 3A-3D depictonly one capsule per electrode pair, multiple capsules may be addressedusing the same electrode pair.

[0062] The smaller electrode 30 is at most one-half the size of thelarger electrode 40. In preferred embodiments the smaller electrode isone-quarter the size of the larger electrode 40; in more preferredembodiments the smaller electrode 30 is one-eighth the size of thelarger electrode 40. In even more preferred embodiments, the smallerelectrode 30 is one-sixteenth the size of the larger electrode 40.

[0063] Causing the particles 50 to migrate to the smaller electrode 30,as depicted in FIG. 3A, allows incident light to pass through thelarger, transparent electrode 40 and be reflected by a reflectingsurface 60. In shutter mode, the reflecting surface 60 is replaced by atranslucent layer, a transparent layer, or a layer is not provided atall, and incident light is allowed to pass through the capsule 20, i.e.the capsule 20 is transmissive.

[0064] Referring now to FIG. 3B, the particles 50 are dispersed into thecapsule 20 by applying an AC field to the capsule 20 via the electrodes30, 40. The particles 50, dispersed into the capsule 20 by the AC field,block incident light from passing through the capsule 20, causing it toappear dark at the viewpoint 10. The embodiment depicted in FIGS. 3A-3Bmay be used in shutter mode by not providing the reflecting surface 60and instead providing a translucent layer, a transparent layer, or nolayer at all. In shutter mode, application of an AC electric fieldcauses the capsule 20 to appear opaque. The transparency of a shuttermode display formed by the apparatus depicted in FIGS. 3A-3D may becontrolled by the number of capsules addressed using DC fields and ACfields. For example, a display in which every other capsule 20 isaddressed using an AC field would appear fifty percent transmissive.

[0065]FIGS. 3C and 3D depict an embodiment of the electrode structuredescribed above in which electrodes 30, 40 are on “top” of the capsule20, that is, the electrodes 30, 40 are between the viewpoint 10 and thecapsule 20. In these embodiments, both electrodes 30, 40 should betransparent. Transparent polymers can be fabricated using conductivepolymers, such as polyaniline, polythiophenes, or indium tin oxide.These materials may be made soluble so that electrodes can be fabricatedusing coating techniques such as spin coating, spray coating, meniscuscoating, printing techniques, forward and reverse roll coating and thelike. In these embodiments, light passes through the electrodes 30, 40and is either absorbed by the particles 50, reflected by retroreflectinglayer 60 (when provided), or transmitted throughout the capsule 20 (whenretroreflecting layer 60 is not provided).

[0066] The addressing structure depicted in FIGS. 3A-3D may be used withelectrophoretic display media and encapsulated electrophoretic displaymedia. FIGS. 3A-3D depict embodiments in which electrode 30, 40 arestatically attached to the display media. In certain embodiments, theparticles 50 exhibit bistability, that is, they are substantiallymotionless in the absence of a electric field. In these embodiments, theelectrodes 30, 40 may be provided as part of a “stylus” or other devicewhich is scanned over the material to address each capsule or cluster ofcapsules. This mode of addressing particle-based displays will bedescribed in more detail below in connection with FIG. 16.

[0067] Referring now to FIGS. 4A and 4B, a capsule 20 of aelectronically addressable media is illustrated in which the techniqueillustrated above is used with multiple rear-addressing electrodes. Thecapsule 20 contains at least one particle 50 dispersed in a clearsuspending fluid 25. The capsule 20 is addressed by multiple smallerelectrodes 30 and multiple larger electrodes 40. In these embodiments,the smaller electrodes 30 should be selected to collectively be at mostone-half the size of the larger electrodes 40. In further embodiments,the smaller electrodes 30 are collectively one-fourth the size of thelarger electrodes 40. In further embodiments the smaller electrodes 30are collectively one-eighth the size of the larger electrodes 40. Inpreferred embodiments, the smaller electrodes 30 are collectivelyone-sixteenth the size of the larger electrodes. Each electrode 30 maybe provided as separate electrodes that are controlled in parallel tocontrol the display. For example, each separate electrode may besubstantially simultaneously set to the same voltage as all otherelectrodes of that size. Alternatively, the electrodes 30, 40 may beinterdigitated to provide the embodiment shown in FIGS. 4A and 4B.

[0068] Operation of the rear-addressing electrode structure depicted inFIGS. 4A and 4B is similar to that described above. For example, thecapsule 20 may contain positively charged, black particles 50 dispersedin a substantially clear suspending fluid 25. The smaller electrodes 30are colored black and the larger electrodes 40 are colored white or arehighly reflective. Referring to FIG. 4A, the smaller electrodes 30 areplaced at a negative potential relative to the larger electrodes 40,causing particles 50 migrate within the capsule to the smallerelectrodes 30 and the capsule 20 appears to the viewpoint 10 as a mix ofthe larger, white electrodes 40 and the smaller, black electrodes 30,creating an effect which is largely white. Referring to FIG. 4B, whenthe smaller electrodes 30 are placed at a positive potential relative tothe larger electrodes 40, particles 50 migrate to the larger electrodes40 causing the capsule 20 to display a mix of the larger, whiteelectrodes 40 occluded by the black particles 50 and the smaller, blackelectrodes 30, creating an effect which is largely black. The techniquesdescribed above with respect to the embodiments depicted in FIGS. 1A and1B for producing two-color displays work with equal effectiveness inconnection with these embodiments.

[0069]FIGS. 5A and 5B depict an embodiment of a rear-addressingelectrode structure that creates a reflective color display in a mannersimilar to halftoning or pointillism. The capsule 20 contains whiteparticles 55 dispersed in a clear suspending fluid 25. Electrodes 42,44, 46, 48 are colored cyan, magenta, yellow, and white respectively.Referring to FIG. 5A, when the colored electrodes 42, 44, 46 are placedat a positive potential relative to the white electrode 48,negatively-charged particles 55 migrate to these three electrodes,causing the capsule 20 to present to the viewpoint 10 a mix of the whiteparticles 55 and the white electrode 48, creating an effect which islargely white. Referring to FIG. 5B, when electrodes 42, 44, 46 areplaced at a negative potential relative to electrode 48, particles 55migrate to the white electrode 48, and the eye 10 sees a mix of thewhite particles 55, the cyan electrode 42, the magenta electrode 44, andthe yellow electrode 46, creating an effect which is largely black orgray. By addressing the electrodes, any color can be produced that ispossible with a subtractive color process. For example, to cause thecapsule 20 to display an orange color to the viewpoint 10, the yellowelectrode 46 and the magenta electrode 42 are set to a voltage potentialthat is more positive than the voltage potential applied by the cyanelectrode 42 and the white electrode 48. Further, the relativeintensities of these colors can be controlled by the actual voltagepotentials applied to the electrodes.

[0070] In another embodiment, depicted in FIG. 6, a color display isprovided by a capsule 20 of size d containing multiple species ofparticles in a clear, dispersing fluid 25. Each species of particles hasdifferent optical properties and possess different electrophoreticmobilities (μ) from the other species. In the embodiment depicted inFIG. 6, the capsule 20 contains red particles 52, blue particles 54, andgreen particles 56, and

|μ_(R)|>|μ_(B)|>|μ_(G)|

[0071] That is, the magnitude of the electrophoretic mobility of the redparticles 52, on average, exceeds the electrophoretic mobility of theblue particles 54, on average, and the electrophoretic mobility of theblue particles 54, on average, exceeds the average electrophoreticmobility of the green particles 56. As an example, there may be aspecies of red particle with a zeta potential of 100 millivolts (mV), ablue particle with a zeta potential of 60 mV, and a green particle witha zeta potential of 20 mV. The capsule 20 is placed between twoelectrodes 32, 42 that apply an electric field to the capsule.

[0072] FIGS. 7A-7B depict the steps to be taken to address the displayshown in FIG. 6 to display a red color to a viewpoint 10. Referring toFIG. 7A, all the particles 52, 54, 56 are attracted to one side of thecapsule 20 by applying an electric field in one direction. The electricfield should be applied to the capsule 20 long enough to attract eventhe more slowly moving green particles 56 to the electrode 34. Referringto FIG. 7B, the electric field is reversed just long enough to allow thered particles 52 to migrate towards the electrode 32. The blue particles54 and green particles 56 will also move in the reversed electric field,but they will not move as fast as the red particles 52 and thus will beobscured by the red particles 52. The amount of time for which theapplied electric field must be reversed can be determined from therelative electrophoretic mobilities of the particles, the strength ofthe applied electric field, and the size of the capsule.

[0073] FIGS. 8A-8D depict addressing the display element to a bluestate. As shown in FIG. 8A, the particles 52, 54, 56 are initiallyrandomly dispersed in the capsule 20. All the particles 52, 54, 56 areattracted to one side of the capsule 20 by applying an electric field inone direction (shown in FIG. 8B). Referring to FIG. 8C, the electricfield is reversed just long enough to allow the red particles 52 andblue particles 54 to migrate towards the electrode 32. The amount oftime for which the applied electric field must be reversed can bedetermined from the relative electrophoretic mobilities of theparticles, the strength of the applied electric field, and the size ofthe capsule. Referring to FIG. 8D, the electric field is then reversed asecond time and the red particles 52, moving faster than the blueparticles 54, leave the blue particles 54 exposed to the viewpoint 10.The amount of time for which the applied electric field must be reversedcan be determined from the relative electrophoretic mobilities of theparticles, the strength of the applied electric field, and the size ofthe capsule.

[0074] FIGS. 9A-9C depict the steps to be taken to present a greendisplay to the viewpoint 10. As shown in FIG. 9A, the particles 52, 54,56 are initially distributed randomly in the capsule 20. All theparticles 52, 54, 56 are attracted to the side of the capsule 20proximal the viewpoint 10 by applying an electric field in onedirection. The electric field should be applied to the capsule 20 longenough to attract even the more slowly moving green particles 56 to theelectrode 32. As shown in FIG. 9C, the electric field is reversed justlong enough to allow the red particles 52 and the blue particles 54 tomigrate towards the electrode 54, leaving the slowly-moving greenparticles 56 displayed to the viewpoint. The amount of time for whichthe applied electric field must be reversed can be determined from therelative electrophoretic mobilities of the particles, the strength ofthe applied electric field, and the size of the capsule.

[0075] In other embodiments, the capsule contains multiple species ofparticles and a dyed dispersing fluid that acts as one of the colors. Instill other embodiments, more than three species of particles may beprovided having additional colors. Although FIGS. 6-9C depict twoelectrodes associated with a single capsule, the electrodes may addressmultiple capsules or less than a full capsule

[0076] In FIG. 10, the rear substrate 100 for a seven segment display isshown that improves on normal rear electrode structures by providing ameans for arbitrarily connecting to any electrode section on the rear ofthe display without the need for conductive trace lines on the surfaceof the patterned substrate or a patterned counter electrode on the frontof the display. Small conductive vias through the substrate allowconnections to the rear electrode structure. On the back of thesubstrate, these vias are connected to a network of conductors. Thisconductors can be run so as to provide a simple connection to the entiredisplay. For example, segment 112 is connected by via 114 through thesubstrate 116 to conductor 118. A network of conductors may run multipleconnections (not shown) to edge connector 122. This connector can bebuilt into the structure of the conductor such as edge connector 122.Each segment of the rear electrode can be individually addressed easilythrough edge connector 122. A continuous top electrode can be used withthe substrate 116.

[0077] The rear electrode structure depicted in FIG. 10 is useful forany display media, but is particularly advantageous for particle-baseddisplays because such displays do not have a defined appearance when notaddressed. The rear electrode should be completely covered in anelectrically conducting material with room only to provide necessaryinsulation of the various electrodes. This is so that the connections onthe rear of the display can be routed with out concern for affecting theappearance of the display. Having a mostly continuous rear electrodepattern assures that the display material is shielded from the rearelectrode wire routing.

[0078] In FIG. 11, a 3×3 matrix is shown. Here, matrix segment 124 on afirst side of substrate 116 is connected by via 114 to conductor 118 ona second side of substrate 116. The conductors 18 run to an edge andterminate in a edge connector 122. Although the display element of FIG.11 shows square segments 124, the segments may be shaped or sized toform a predefined display pattern.

[0079] In FIG. 12, a printed circuit board 138 is used as the rearelectrode structure. The front of the printed circuit board 138 hascopper pads 132 etched in the desired shape. There are plated vias 114connecting these electrode pads to an etched wire structure 136 on therear of the printed circuit board 138. The wires 136 can be run to oneside or the rear of the printed circuit board 138 and a connection canbe made using a standard connector such as a surface mount connector orusing a flex connector and anisotropic glue (not shown). Vias may befilled with a conductive substance, such as solder or conductive epoxy,or an insulating substance, such as epoxy.

[0080] Alternatively, a flex circuit such a copper-clad polyimide may beused for the rear electrode structure of FIG. 10. Printed circuit board138 may be made of polyimide, which acts both as the flex connector andas the substrate for the electrode structure. Rather than copper pads132, electrodes (not shown) may be etched into the copper covering thepolyimide printed circuit board 138. The plated through vias 114 connectthe electrodes etched onto the substrate the rear of the printed circuitboard 138, which may have an etched conductor network thereon (theetched conductor network is similar to the etched wire structure 136).

[0081] In FIG. 12, a thin dielectric sheet 150, such as polyester,polyimide, or glass can be used to make a rear electrode structure.Holes 152 are punched, drilled, abraded, or melted through the sheetwhere conductive paths are desired. The front electrode 154 is made ofconductive ink printed using any technique described above. The holesshould be sized and the ink should be selected to have a viscosity sothat the ink fills the holes. When the back structure 156 is printed,again using conductive ink, the holes are again filled. By this method,the connection between the front and back of the substrate is madeautomatically.

[0082] In FIG. 14, the rear electrode structure can be made entirely ofprinted layers. A conductive layer 166 can be printed onto the back of adisplay comprised of a clear, front electrode 168 and a printabledisplay material 170. A clear electrode may be fabricated from indiumtin oxide or conductive polymers such as polyanilines andpolythiophenes. A dielectric coating 176 can be printed leaving areasfor vias. Then, the back layer of conductive ink 178 can be printed. Ifnecessary, an additional layer of conductive ink can be used before thefinal ink structure is printed to fill in the holes.

[0083] This technique for printing displays can be used to build therear electrode structure on a display or to construct two separatelayers that are laminated together to form the display. For example anelectronically active ink may be printed on an indium tin oxideelectrode. Separately, a rear electrode structure as described above canbe printed on a suitable substrate, such as plastic, polymer films, orglass. The electrode structure and the display element can be laminatedto form a display.

[0084] Referring now to FIG. 15, a threshold may be introduced into anelectrophoretic display cell by the introduction of a third electrode.One side of the cell is a continuous, transparent electrode 200 (anode).On the other side of the cell, the transparent electrode is patternedinto a set of isolated column electrode strips 210. An insulator 212covers the column electrodes 210, and an electrode layer on top of theinsulator is divided into a set of isolated row electrode strips 230,which are oriented orthogonal to the column electrodes 210. The rowelectrodes 230 are patterned into a dense array of holes, or a grid,beneath which the exposed insulator 212 has been removed, forming amultiplicity of physical and potential wells.

[0085] A positively charged particle 50 is loaded into the potentialwells by applying a positive potential (e.g. 30V) to all the columnelectrodes 210 while keeping the row electrodes 230 at a less positivepotential (e.g. 15V) and the anode 200 at zero volts. The particle 50may be a conformable capsule that situates itself into the physicalwells of the control grid. The control grid itself may have arectangular cross-section, or the grid structure may be triangular inprofile. It can also be a different shape which encourages themicrocapsules to situate in the grid, for example, hemispherical.

[0086] The anode 200 is then reset to a positive potential (e.g. 50V).The particle will remain in the potential wells due to the potentialdifference in the potential wells: this is called the Hold condition. Toaddress a display element the potential on the column electrodeassociated with that element is reduced, e.g. by a factor of two, andthe potential on the row electrode associated with that element is madeequal to or greater than the potential on the column electrode. Theparticles in this element will then be transported by the electric fielddue to the positive voltage on the anode 200. The potential differencebetween row and column electrodes for the remaining display elements isnow less than half of that in the normal Hold condition. The geometry ofthe potential well structure and voltage levels are chosen such thatthis also constitutes a Hold condition, i.e., no particles will leavethese other display elements and hence there will be no half-selectproblems. This addressing method can select and write any desiredelement in a matrix without affecting the pigment in any other displayelement. A control electrode device can be operated such that the anodeelectrode side of the cell is viewed.

[0087] The control grid may be manufactured through any of the processesknown in the art, or by several novel processes described herein. Thatis, according to traditional practices, the control grid may beconstructed with one or more steps of photolithography and subsequentetching, or the control grid may be fabricated with a mask and a“sandblasting” technique.

[0088] In another embodiment, the control grid is fabricated by anembossing technique on a plastic substrate. The grid electrodes may bedeposited by vacuum deposition or sputtering, either before or after theembossing step. In another embodiment, the electrodes are printed ontothe grid structure after it is formed, the electrodes consisting of somekind of printable conductive material which need not be clear (e.g. ametal or carbon-doped polymer, an intrinsically conducting polymer,etc.).

[0089] In a preferred embodiment, the control grid is fabricated with aseries of printing steps. The grid structure is built up in a series ofone or more printed layers after the cathode has been deposited, and thegrid electrode is printed onto the grid structure. There may beadditional insulator on top of the grid electrode, and there may bemultiple grid electrodes separated by insulator in the grid structure.The grid electrode may not occupy the entire width of the gridstructure, and may only occupy a central region of the structure, inorder to stay within reproducible tolerances. In another embodiment, thecontrol grid is fabricated by photoetching away a glass, such as aphotostructural glass.

[0090] In an encapsulated electrophoretic image display, anelectrophoretic suspension, such as the ones described previously, isplaced inside discrete compartments that are dispersed in a polymermatrix. This resulting material is highly susceptible to an electricfield across the thickness of the film. Such a field is normally appliedusing electrodes attached to either side of the material. However, asdescribed above in connection with FIGS. 3A-3D, some display media maybe addressed by writing electrostatic charge onto one side of thedisplay material. The other side normally has a clear or opaqueelectrode. For example, a sheet of encapsulated electrophoretic displaymedia can be addressed with a head providing DC voltages.

[0091] In another implementation, the encapsulated electrophoreticsuspension can be printed onto an area of a conductive material such asa printed silver or graphite ink, aluminized mylar, or any otherconductive surface. This surface which constitutes one electrode of thedisplay can be set at ground or high voltage. An electrostatic headconsisting of many electrodes can be passed over the capsules toaddressing them. Alternatively, a stylus can be used to address theencapsulated electrophoretic suspension.

[0092] In another implementation, an electrostatic write head is passedover the surface of the material. This allows very high resolutionaddressing. Since encapsulated electrophoretic material can be placed onplastic, it is flexible. This allows the material to be passed throughnormal paper handling equipment. Such a system works much like aphotocopier, but with no consumables. The sheet of display materialpasses through the machine and an electrostatic or electrophotographichead addresses the sheet of material.

[0093] In another implementation, electrical charge is built up on thesurface of the encapsulated display material or on a dielectric sheetthrough frictional or triboelectric charging. The charge can built upusing an electrode that is later removed. In another implementation,charge is built up on the surface of the encapsulated display by using asheet of piezoelectric material.

[0094]FIG. 16 shows an electrostatically written display. Stylus 300 isconnected to a positive or negative voltage. The head of the stylus 300can be insulated to protect the user. Dielectric layer 302 can be, forexample, a dielectric coating or a film of polymer. In otherembodiments, dielectric layer 302 is not provided and the stylus 300contacts the encapsulated electrophoretic display 304 directly.Substrate 306 is coated with a clear conductive coating such as ITOcoated polyester. The conductive coating is connected to ground. Thedisplay 304 may be viewed from either side.

[0095] Microencapsulated displays offer a useful means of creatingelectronic displays, many of which can be coated or printed. There aremany versions of microencapsulated displays, including microencapsulatedelectrophoretic displays. These displays can be made to be highlyreflective, bistable, and low power.

[0096] To obtain high resolution displays, it is useful to use someexternal addressing means with the microencapsulated material. Thisinvention describes useful combinations of addressing means withmicroencapsulated electrophoretic materials in order to obtain highresolution displays.

[0097] One method of addressing liquid crystal displays is the use ofsilicon-based thin film transistors (TFTs) to form an addressingbackplane for the liquid crystal. For liquid crystal displays, thesethin film transistors are typically deposited on glass, and aretypically made from amorphous silicon or polysilicon. Other electroniccircuits (such as drive electronics or logic) are sometimes integratedinto the periphery of the display. An emerging field is the depositionof amorphous or polysilicon devices onto flexible substrates such asmetal foils or plastic films.

[0098] The addressing electronic backplane could incorporate diodes asthe nonlinear element, rather than transistors. Diode-based activematrix arrays have been demonstrated as being compatible with liquidcrystal displays to form high resolution devices.

[0099] There are also examples of crystalline silicon transistors beingused on glass substrates. Crystalline silicon possesses very highmobilities, and thus can be used to make high performance devices.Presently, the most straightforward way of constructing crystallinesilicon devices is on a silicon wafer. For use in many types of liquidcrystal displays, the crystalline silicon circuit is constructed on asilicon wafer, and then transferred to a glass substrate by a “liftoff”process. Alternatively, the silicon transistors can be formed on asilicon wafer, removed via a liftoff process, and then deposited on aflexible substrate such as plastic, metal foil, or paper. As anotherimplementation, the silicon could be formed on a different substratethat is able to tolerate high temperatures (such as glass or metalfoils), lifted off, and transferred to a flexible substrate. As yetanother implementation, the silicon transistors are formed on a siliconwafer, which is then used in whole or in part as one of the substratesfor the display.

[0100] The use of silicon-based circuits with liquid crystals is thebasis of a large industry. Nevertheless, these display possess seriousdrawbacks. Liquid crystal displays are inefficient with light, so thatmost liquid crystal displays require some sort of backlighting.Reflective liquid crystal displays can be constructed, but are typicallyvery dim, due to the presence of polarizers. Most liquid crystal devicesrequire precise spacing of the cell gap, so that they are not verycompatible with flexible substrates. Most liquid crystal displaysrequire a “rubbing” process to align the liquid crystals, which is bothdifficult to control and has the potential for damaging the TFT array.

[0101] The combination of these thin film transistors withmicroencapsulated electrophoretic displays should be even moreadvantageous than with liquid crystal displays. Thin film transistorarrays similar to those used with liquid crystals could also be usedwith the microencapsulated display medium. As noted above, liquidcrystal arrays typically requires a “rubbing” process to align theliquid crystals, which can cause either mechanical or static electricaldamage to the transistor array. No such rubbing is needed formicroencapsulated displays, improving yields and simplifying theconstruction process.

[0102] Microencapsulated electrophoretic displays can be highlyreflective. This provides an advantage in high-resolution displays, as abacklight is not required for good visibility. Also, a high-resolutiondisplay can be built on opaque substrates, which opens up a range of newmaterials for the deposition of thin film transistor arrays.

[0103] Moreover, the encapsulated electrophoretic display is highlycompatible with flexible substrates. This enables high-resolution TFTdisplays in which the transistors are deposited on flexible substrateslike flexible glass, plastics, or metal foils. The flexible substrateused with any type of thin film transistor or other nonlinear elementneed not be a single sheet of glass, plastic, metal foil, though.Instead, it could be constructed of paper. Alternatively, it could beconstructed of a woven material. Alternatively, it could be a compositeor layered combination of these materials.

[0104] As in liquid crystal displays, external logic or drive circuitrycan be built on the same substrate as the thin film transistor switches.

[0105] In another implementation, the addressing electronic backplanecould incorporate diodes as the nonlinear element, rather thantransistors.

[0106] In another implementation, it is possible to form transistors ona silicon wafer, dice the transistors, and place them in a large areaarray to form a large, TFT-addressed display medium. One example of thisconcept is to form mechanical impressions in the receiving substrate,and then cover the substrate with a slurry or other form of thetransistors. With agitation, the transistors will fall into theimpressions, where they can be bonded and incorporated into the devicecircuitry. The receiving substrate could be glass, plastic, or othernonconductive material. In this way, the economy of creating transistorsusing standard processing methods can be used to create large-areadisplays without the need for large area silicon processing equipment.

[0107] While the examples described here are listed using encapsulatedelectrophoretic displays, there are other particle-based display mediawhich should also work as well, including encapsulated suspendedparticles and rotating ball displays.

[0108] While the invention has been particularly shown and describedwith reference to specific preferred embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention as defined by the appended claims.

What is claimed is:
 1. An electrophoretic display comprising: asubstrate; at least one capsule containing a suspending fluid and atleast one charged particle, said charged particle having an opticalproperty; and at least two electrodes disposed on said substrateadjacent said at least one capsule and positioned in a spaced apartrelationship to one another, wherein a potential difference between saidelectrodes causes the particles to migrate toward at least one of saidat least two electrodes, thereby effecting change in visual state. 2.The display of claim 1, wherein said suspending fluid is substantiallytransparent.
 3. The display of claim 1, wherein said at least onecharged particle has a black color.
 4. The display of claim 1, whereinsaid at least one charged particle has a white color.
 5. The display ofclaim 1, wherein one of said at least two electrodes is substantiallytransparent.
 6. The display of claim 1, wherein both of said at leasttwo electrodes are substantially transparent.
 7. The display of claim 1,wherein one of said at least two electrodes has a first optical propertyand the other of said at least two electrodes has a second opticalproperty.
 8. The display of claim 6, wherein said at least one chargedparticle is black and wherein application of a first voltage potentialto said black electrode causes said black particles to migrate withinsaid capsule to a location adjacent said black electrode, causing saidcapsule to appear substantially white, and wherein application of asecond voltage potential to said black electrode causes said blackparticles to migrate within said capsule to a location adjacent saidwhite electrode causing said capsule to appear substantially black. 9.An electrophoretic display comprising: at least one capsule containing asuspending fluid and at least one particle having a first opticalproperty; at least two electrodes, each having a second optical propertyadjacent said at least one capsule; and at least one electrode havingsaid first optical property adjacent said at least one capsule; whereinapplication of a voltage potential to said at least two electrodescauses the capsule to change visual state.
 10. The electrophoreticdisplay of claim 8, wherein each of said at least two electrodes has adifferent optical property.
 11. The electrophoretic display of claim 8,wherein said suspending fluid is dyed.
 12. The electrophoretic displayof claim 8, wherein said suspending fluid is substantially transparent.13. The electrophoretic display of claim 8 wherein said at least oneparticle has a black color.
 14. The electrophoretic display of claim 8wherein said at least one electrode is substantially transparent.